Palaeoproterozoic metavolcanic and metasedimentary succession hostingthe Dannemora iron ore deposits, Bergslagen region, Sweden
2012 (English)In: GFF, ISSN 1103-5897, Vol. 134, no 2, 71-85 p.Article in journal (Refereed) Published
The Dannemora supracrustal inlier is located in the north-eastern part of the Bergslagen regionin south-central Sweden and hosts the second largest iron ore deposit in the region. The metasupracrustalsuccession of the inlier consists of c. 1.9 Ga Palaeoproterozoic rocks that are mainly sub-alkaline, rhyoliticto dacitic, pyroclastic deposits, reworked pyroclastic deposits and metalimestone. It is c. 700–800-m thickand termed the Dannemora Formation. The formation is divided into lower and upper members and theformer is in turn subdivided into subunits 1 and 2. The great thickness of individual pyroclastic depositsindicates deposition within a caldera. The rocks show characteristics of a pyroclastic origin by containingabundant pumice, cuspate and Y-shaped former glass shards, and fragmented crystals of quartz andsubordinate feldspars. Scattered spherulites and lack of welding-compacted fiamme suggest that the lowermember was slightly welded, where as the upper member contains sericite-replaced glass shards withpreserved primary shapes indicating no welding. Undisturbed layers of ash-siltstone with normal gradingand fluid–escape structures are attributed to subaqueous deposition below storm wave base in the easternpart of the inlier, where as erosion channels and cross-bedding in some of the volcaniclastic deposits implydeposition and reworking above wave base in the central part of the inlier. Epidote spots, previouslyinterpreted as altered limestone fragments and an indicator for subaquatic deposition, are here reinterpretedas the result of selective alteration related to the intrusion of mafic dykes and to Ca release duringdolomitisation of limestone.
Place, publisher, year, edition, pages
Taylor & Francis, 2012. Vol. 134, no 2, 71-85 p.
Bergslagen; Dannemora; Palaeoproterozoic; pyroclastic rocks; volcanic textures; ignimbrites
IdentifiersURN: urn:nbn:se:uu:diva-182809DOI: 10.1080/11035897.2012.674551OAI: oai:DiVA.org:uu-182809DiVA: diva2:560792